There’s no story that better illustrates the power of the growing anti-Trump grassroots resistance movement than Representative Rodney Frelinghuysen’s about-face on the president’s health-care bill. Frelinghuysen’s defection was a major victory for a new liberal grassroots movement that sprung up in his affluent New Jersey district soon after Donald Trump’s surprise November victory, and pressured him to change his vote.

As chair of the House Appropriations Committee, Frelinghuysen is a key member of House Speaker Paul Ryan’s leadership team. The New Jersey Republican has been an important ally on the president’s policy initiatives. Like other party stalwarts, he repeatedly pledged to support Trump and Ryan’s Obamacare “repeal and replace” bill. But just hours before Ryan scheduled a vote on their American Health Care Act last Friday, Frelinghuysen threw the administration a curveball. Calling the legislation “unacceptable,” Frelinghuysen said in a statement that he planned to vote against it.

His change of heart doomed the bill’s prospects, signaling to other House Republicans that they could defy Ryan’s leadership and Trump’s threats to punish them. Acknowledging defeat, Ryan pulled the bill before Republican House members had to cast their votes. Overall, 33 Republicans brought the bill down. Until this bill, Frelinghuysen had fallen in line with Trump and Ryan on every vote during this congressional session.

The burgeoning movement, spearheaded by the NJ 11th for Change coalition, kept the heat on Frelinghuysen with weekly protests against the bill and a not-so-veiled threat to run a Democrat against him next year. Upset that Frelinghuysen hadn’t held an in-person town hall meeting in three years, NJ 11th for Change also sponsored five “Where’s Rodney?” town hall gatherings in Morristown, Little Falls, Sparta, Livingston, and Boonton—and invited the congressman.

He didn’t show up to any of them. But each time, hundreds of voters did, and organizers responded to Frelinghuysen’s absence with a cardboard cutout cartoon likeness of the 70-year-old “seated” in a chair. Stung by the group’s success in drawing media attention, but still unwilling to attend in-person meetings with his constituents, Frelinghuysen instead held several lightly publicized “telephone town halls” in March. The night before the scheduled vote, NJ 11th for Change staged a symbolic seventh birthday party for Obamacare outside Frelinghuysen’s Morristown office.

After Frelinghuysen’s announcement about his “no” vote, NJ 11th for Change issued a statement explaining that it had “organized a sustained effort with thousands of our members—calling, visiting, mailing, faxing, and showing up [at Frelinghuysen's office]— all urging him to vote against taking health care away from more than 30,000 fellow constituents.”

Montclair resident David Rosenberg created the NJ 11th for Change Facebook group one week after the presidential election, “after the shock wore off,” he says. At the outset, there were only three members, but that number skyrocketed to 7,000 by early March, and included many independents and Republicans.

“When we started, I thought we’d have ten people sitting around, having coffee, bitching and moaning,” Rosenberg tells The American Prospect. “But people are coming out of the woodwork. They’re saying, ‘I want in. How can I help?’” He adds, “Our mission is to hold Rodney accountable and let people know where he stands.”

Since January, the group has organized weekly “Fridays with Frelinghuysen” protests outside his Morristown district office. The group gathers at a nearby Starbucks before walking over to Frelinghuysen's office, bringing pastries for his office staff and messages opposing his stances on key issues. As many as 400 people have shown up.

Frelinghuysen’s support for “repeal and replace” was only one of the issues that brought hundreds of people to NJ 11th for Change’s protests and monthly meetings. The group has challenged him on immigration, the elimination of federal funding for Planned Parenthood, the overturning of environmental regulations, and the loosening of restrictions on gun purchases by people with mental illnesses. Organizers have also joined forces with Planned Parenthood, the Service Employees International Union (SEIU), the Working Families Party, New Jersey Citizen Action, Clean Water Action, and Blue Wave New Jersey, a coalition of progressive activists.

Most of NJ 11th for Change’s members are political neophytes, according to Rosenberg, himself a newcomer to activism. The group arranged a Skype session to discuss strategy with Indivisible founder Ezra Levin, whose “how to” guide for protesting against and defeating Republicans has spawned more than 5,000 local groups around the country.

“We’ve all read the Indivisible manual,” Rosenberg says. “We know we have to apply the Tea Party tactics, but be more respectful. We’re angry and fed up and we’re making ourselves known. Frelinghuysen is out of touch with the people in our district.” The coalition plans to educate voters about Frelinghuysen’s record and to register new voters.

Frelinghuysen is the scion of a prominent New Jersey family whose history dates to back to the Revolutionary War. He is the sixth member of his family to serve in Congress. Frederick Frelinghuysen, his great-great-great-great-grandfather, was one of the framers of New Jersey’s first constitution and served in the U.S Senate. His father, Peter Frelinghuysen Jr., also a Republican, served in Congress from 1953 to 1975, representing much of the same affluent suburban Morris County and its environs that his son does today. One of the wealthiest members of Congress, Rodney Frelinghuysen is the heir to the Ballantine Brewery and Procter & Gamble fortunes.

Between 1994 and 2014, Frelinghuysen won between 58.8 percent and 72 percent of the vote in his district. But his victory last year, with 58 percent of the vote, was his lowest margin ever. Trump, meanwhile, beat Hillary Clinton by less than 1 percentage point—48.8 percent to 47.9 percent—in the district.

For most of his congressional career, Frelinghuysen was a moderate Republican, but in recent years has moved to the right. He and his wife used to donate money to their local Planned Parenthood chapter, and he voted to fund Planned Parenthood in 2007, 2009, and 2011. But in 2015, he began voting against funding.

Frelinghuysen only belatedly realized that backing Trump had eroded his support in the district, which led to his speedy U-turn on health care. Long considered a safe Republican stronghold, the Democratic Congressional Campaign Committee (DCCC) put the 11th District, along with three other New Jersey House seats currently held by Republicans, on its target list of 59 battleground districts where the party intends to make a stand in 2018. West of New York City, the 11th, with a per capita income of $48,898, is one of the wealthiest congressional districts in the country.

Democrats hope to exploit the backlash against Trump to defeat Republican House members who sided with the president. To neutralize most of Trump’s initiatives, the Democrats need to gain 24 seats to win a House majority. No Democrat has emerged yet to run for the seat, but Rosenberg is confident that several will compete in next year’s June party primary. “This is now a swing district,” he says, “and it can be swung to the Democratic side.”